1. Field of the Invention
The present invention relates to a method for laying and joining pipes, especially large concrete pipes, for forming a pipe line composed of many pipes, by the aid of a two-legged lifting strap. The invention also concerns a lifting device for carrying out the method.
2. Description of the Prior Art
When heavy pipes, such as sewer pipes made of concrete, are to be mounted onto a pipe line in a trench, each pipe is usually hoisted by means of a special tool in the form of a grab hook, a lifting yoke is inserted into the pipe, such as a wire sling or the like, and the pipe is moved to position in the trench adjacent the end of the line of previously laid pipes. The joint end of a new pipe which is often provided with a rubber gasket is pushed into the socket of the previously laid pipe. Since the pipes are often very heavy and the friction in the joint is very great due to the rubber gasket, a considerable force is necessary to effect the joining of the pipes. Another problem is that the new pipe has to be connected to the previously laid pipe without the latter being moved out of its position, which may easily happen if the previously laid pipe is used for "anchoring" the force used for pushing the new pipe into position. For this reason, it is necessary to effect the joining of the pipes by the aid of a further special tool which pulls or pushes the new pipe into the pipe line by means of a lever mechanism or by hydraulic means.
The special tools used for laying the pipes as well as the ones used for joining the pipes are usually heavy, bulky and complicated and therefore also expensive to purchase and maintain, so that it is often impossible for contractors to acquire tools of their own. Furthermore, the change from the hoisting tool to the joining tool is time consuming and there is a risk that previously laid pipes will be moved out of position in connection with the mounting of a new pipe.